Develop Reference

retrieving POJO objects programmatically?

Let, say for instance i have a POJO class employee with three attributes
1.Name (String)
2.Location (String)
3.Date of Birth (Date)
then i fired a query into database which retrieve first row of table and populate this POJO
with table data as follows:-
Name - john
location - USA
Date of Birth - 27/09/2014
To retrieve the values from this POJO i have to call getName(),getLocation() and getDOB().
But is there any method by which i can get all the values which is store in the POJO, in an Object type array without using getter method
for example:
Object[0] has the value "John".
Object[1] has the value "USA".
Object[2] has the value "27/09/2014".
(In my case, there are around 80 attributes in a class and number of these attributes increases because of client requirements and i am fetching each and every value by getter method and every time a single attribute is added i have to write a getter method in the code to fetch values. I basically want a more dynamic solution to this problem.)

you can try this:
String[] getObjectsPublicMethods(Object o)
{
Class clazz = o.getClass();
Method[] methods = clazz.getDeclaredMethods();
String[] result = new String[methods.length];
for (int i=0; i<methods.length; ++i)
{
try
{
result[i] = (String) methods[i].invoke(o, new Object[] {})
} catch (IllegalAccessException e)
{
e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates.
} catch (InvocationTargetException e)
{
}
}
return result;
}
This method uses reflection to get the information you want, BUT
it assumes that the getter methods are declared public in this class AND
that all of the public methods return String.

I think what you are looking for is called reflection.
i hope this link helps http://docs.oracle.com/javase/tutorial/reflect/index.html
or this answer:
Is it possible to use Java Reflection to print out attributes of the parent class?

How to get the string value of a parameter in Java?

I'm trying to save a parameter as a property, the parameter is : testSpeed ( = "20" ), and I have a method :
saveProperty(String Parameter)
which will save the value of Parameter to a file, something like this :
saveProperty(testSpeed);
My property file looks like this :
testSpeed : 20
testHeight : 300
My question is : In Java is it possible to get the name of the parameter, in this case the parameter passed to saveProperty() is testSpeed, and it's name is "testSpeed", how to get this string from inside of saveProperty(), so I don't have to do the following to save it :
saveProperty(String Name,String Value);
and call it like this :
saveProperty("testSpeed",testSpeed)
Also, when I need to get it out from the property file, I can call :
getProperty(testSpeed)
I know how to use a property file, I'm just using it as an example, maybe it caused some confusion when I mentioned property file.
The essence of the question is : how to get the name of a parameter passed into a method.
void someMethod(String Parameter_XYZ)
{
// In here if I call someMethod(testSpeed), how to get the string "testSpeed",
// not it's value, but it's name,
// exactly spelled as "testSpeed" ?
}
Is this doable in Java ?

First of all correct format of property file is this:
testSpeed=20
testHeight=300
Now to pull any property from this you need to pass key name, which you're already doing in saveProperty(String name, String value) method. However if I understood your question correctly then you want to avoid passing property name. You can have some custom methods like this:
void saveTestSpeed(String value) {
saveProperty("testSpeed", value);
}
Strig getTestSpeed() {
getProperty("testSpeed");
}
EDIT: Based on your edited question.
No it is not possible in Java.
Please remember that Java is strictly pass-by-value that makes it impossible to figure out the name of actual variable name at callee's end. Inside the saveProperty method we can only get the value of the arguments since in Java Object references are passed by value not by name.
This is not practical also since there is nothing that stops method being called like:
savePropery( "20" );
In the case what should be the name of the property?

Use Properties:
public class Util
{
private static final Properties prop = new Properties();
static
{
try
{
prop.load(new FileReader(new File("prop.txt")));
}
catch(IOException e)
{
e.printStackTrace();
}
}
public static void setProperty(String key, String value)
{
prop.setProperty(key, value);
persistCurrentProperites();
}
public static String getProperty(String key)
{
return prop.getProperty(key);
}
private static void persistCurrentProperites()
{
try
{
prop.store(new FileWriter(new File("prop.txt")), null);
}
catch(IOException e)
{
e.printStackTrace();
}
}
}

I suggest maintaining the key-value pairs to acheive a more generic solution. You can use Properties
void saveProperty(String key, String value) {
properties.setProperty(key, value);
}
use getProperty to retrieve
properties.getProperty(key)

Convert an attribute to its name

It is possible to access a class attribute by its name using the following code:
MyClass.class.getDeclaredField("myAtt");
is it possible to create a function which do reverse? i.e. a function which convert an attribute to its name?
EDIT: I'm adding this edit to make this more clear:
consider the following class:
class MyClass {
Integer myAttribute;
}
I am looking for a function which accepts myAttribute (itself, or a reference to it. idk!) and returns "myAttribute" string.

Try java.lang.reflect.Field.getName();

You can use Field#getName method : -
Field[] fields = MyClass.class.getDeclaredFields();
for (Field field: fields) {
System.out.println(field.getName());
}
PS: - You should name your classes starting with uppercase letters.
UPDATE: - Ok, if you have your name of the attribute: - private String attrName;, you can get the corresponding field my using: -
Field field = MyClass.class.getDeclaredField("attrName");
// Then do this. Which is meaningless. But I don't know why you want to do this
// Your fieldName will contain "attrName" which you know already
String fieldName = field.getName();
But fetching the name from the above field doesn't make sense. I would be surprised if you were looking for this only. But you can be more clear with your question.

If I understand your question you want to get the name of the Field thats current value matches the value of a variable.
private String getAttributeName(Object myInstance, Object myValue) throws IllegalAccessException
{
Field[] fields = MyClass.class.getDeclaredFields();
for (Field each : fields)
{
if (each.get(myInstance).equals(myValue))
return each.getName();
}
return null;
}
Then you can call it,
getAttributeName(myInstance, myValue);

NoSuchFieldException on getClass().getField()

java.lang.NoSuchFieldException: id
The below line is creating the exception.
String fieldValue =String.valueOf(studyplanCategory.getClass().getField(filterProperty).get(studyplanCategory));
studyplanCategory is a valid object and has got actual values. Beacuse of this exception the load method and the search function in the LazyLoading DataTable of my JSF webapp is not working.

From the Javadoc for Class.getField(...):
Returns a Field object that reflects the specified public member field
of the class or interface represented by this Class object. The name
parameter is a String specifying the simple name of the desired field.
The field to be reflected is determined by the algorithm that follows.
Let C be the class represented by this object:
If C declares a public field with the name specified, that is the
field to be reflected. If no field was found in step 1 above, this
algorithm is applied recursively to each direct superinterface of C.
The direct superinterfaces are searched in the order they were
declared. If no field was found in steps 1 and 2 above, and C has a
superclass S, then this algorithm is invoked recursively upon S. If C
has no superclass, then a NoSuchFieldException is thrown. See The Java
Language Specification, sections 8.2 and 8.3.
If the field you are trying to retrieve via:
studyplanCategory.getClass().getField(filterProperty)
is private, then you will get a NoSuchFieldException. For private fields, try this instead:
studyplanCategory.getClass().getDeclaredField(filterProperty)
And to get around potential illegal access exceptions when setting values via a field this way:
Field field = studyplanCategory.getClass().getDeclaredField(filterProperty);
field.setAccessible(true);
field.get(studyplanCategory);

App fires up this exception because its doesn't see attribudes your want to give back.
Method getField() return non-private attribudes so if your attribudes are private, method doesn't see them. You can check http://docs.oracle.com/javase/tutorial/reflect/member/fieldTrouble.html
So you can do that your attribudes will change on protected or public and then should work it right. But this way (its same like example on primefaces) simulate real database.
public List<Car> load(int first, int pageSize, String sortField, SortOrder sortOrder, Map<String,String> filters) {
List<Car> data = new ArrayList<Car>();
//filter
for(Car car : datasource) {
boolean match = true;
for(Iterator<String> it = filters.keySet().iterator(); it.hasNext();) {
try {
String filterProperty = it.next();
String filterValue = filters.get(filterProperty);
String fieldValue = String.valueOf(car.getClass().getField(filterProperty).get(car));
...
So this list simulate real database only for example. If you want to use it. so you shoud do it on backing bean class and there do it. You open connection already with some filter or don't and then return data from database.
//EDIT: Man wrote that you should use getDeclaredField() but i did try this and it didn't work well, and throws up IlegalAccessException. When a pretype attribudes to protected, it works fine. I don't know why.

Best solutions for getClass().getField() problem are:
Use getDeclaredField() instead of getField()
1)
String propertyName = "test";<br/>
Class.forName(this.getClass().getName()).getDeclaredField(propertyName);
2)
String propertyName = "name";<br/>
Replace **"HelloWorld"** with your class name<br/>
HelloWorld.class.getDeclaredField(propertyName)

I had faced the same problem. My issue was my variables are not public . Make sure your class variables are public
private Object getVariableValue(Object clazz, String variableName) {
try {
Field field = clazz.getClass().getField(variableName);
return field.get(clazz);
} catch (NoSuchFieldException e) {
return "";
} catch (IllegalAccessException e) {
return "";
}
}

I agree that we should Use getDeclaredField() instead of getField()
private Field getOwnProperty(Object clazz, String propertyName) {
try {
return clazz.getClass().getDeclaredField(propertyName);
} catch (NoSuchFieldException e) {
log.warn( "Object has no property : " + propertyName );
return null;
}
}

How to know whether a property exists or not in a property file?

How to know whether a property exists or not in a property file in java?

According to http://java.sun.com/javase/6/docs/api/java/util/Properties.html, getProperty() returns null if the property was not found. You could also call propertyNames() or stringPropertyNames() and look to see whether the property name of interest is in the returned set.

Yet another alternative is to exploit the fact the Properties extends Hashtable<Object,Object> and use containsKey.

Just load the properties file and then try to get the desired property.
public String getProperty(String key)
Searches for the property with the specified key in this property list. If the key is not found in this property list, the default property list, and its defaults, recursively, are then checked. The method returns null if the property is not found.
http://java.sun.com/j2se/1.5.0/docs/api/java/util/Properties.html#getProperty(java.lang.String)

You can also call getProperty(String key, String defaultValue) and check if the return value is the defaultValue.
See https://docs.oracle.com/javase/8/docs/api/java/util/Properties.html#getProperty-java.lang.String-java.lang.String-

You can use hasProperty
AllValues.hasProperty("childList")

If you want to check that at the start of program you can do the following:
Create a class VerifiedProperties that extends Properties
Add all properties as fields of this class as public final int/String/boolean/etc...
Add private final String propertyNotValid="Property not valid" String to this class
Add private final String propertyNotFound="Property not found" String to this class
Override getProperty() method from Properties class.
You can add #Deprecated tag to suggest usage of the fields. It is impossible to hide this method because it is public in Properties class.
Initialize all fields in the constructor using getProperty() method or dedicated for type (look examples below)
Example methods that takes care of different property types:
#Override
#Deprecated
/*
Deprecated annotation added to suggest usage of the fields.
*/
public final String getProperty(String key)
{
String propertyValue = super.getProperty(key);
if (propertyValue != null)
{
return propertyValue;
}
else
{
throw new NoSuchFieldError(this.propertyNotFound + " " + key);
}
}
private int getIntegerProperty(String key)
{
String propertyValue = this.getProperty(key);
try
{
int propertyIntValue = Integer.parseInt(propertyValue);
return propertyIntValue;
}
catch (NumberFormatException e)
{
throw new NumberFormatException(this.propertyNotValid + " " + key);
}
}
private boolean getBooleanProperty(String key)
{
String propertyValue = this.getProperty(key);
try
{
boolean propertyBooleanValue = Boolean.parseBoolean(propertyValue);
return propertyBooleanValue;
}
catch (NumberFormatException e)
{
throw new NumberFormatException(this.propertyNotValid + " " + key);
}
}
private long getLongProperty(String key)
{
String propertyValue = this.getProperty(key);
try
{
long propertyLongValue = Long.parseLong(propertyValue);
return propertyLongValue;
}
catch (NumberFormatException e)
{
throw new NumberFormatException(this.propertyNotValid + " " + key);
}
}
Then you can create somewhere:
public static VerifiedProperties properties;
and use the properties that you need as properties.myProperty
Advantages:
you have full control over properties which includes exception handling and null checking
If property does not exist or is in incorrect format, you will have the information at the initialization of the program
You don't need to worry about parsing properties to different types than String in your code.
You can add validators to your String properties
You can easily refactor property name
If you are using external property file that can be modified by the user outside of your application, if provided change is incorrect or there are fields missing your program will not start.
Disadvantages:
For each property besides adding value to *.properties file you need to create field and assign value in the constructor. If you have a lot of properties then this file can look unpleasant.
Hints:
it is easier to mantain the file if you keep the same name for the field as in properties file.
(Netbeans) you can Toggle Rectangular Selection to add public final String and similar to many lines at once.
(Netbeans) to keep *.properties file clean you can use this solution.

The answer by crazyscot is now outdated. According to the new javadoc, the property will just get created if it doesn't exist,
"If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties method".

Here is some trick how to find out is some file (not mandatory property file) exists in class path
public class FileUtil {
public static boolean isFileExists(String fileName){
return null != FileUtil.class.getResourceAsStream(fileName);
}
}
Sure it not always works as long it depends on class loading aspects